Khalid Sheikh Mohammed, the al-Qaida ringleader who claims to have masterminded the 9/11 attacks, was held, interrogated, and possibly tortured at a secret CIA prison in Poland, the leading investigator of the US policy of "extraordinary renditions" said yesterday.

Unveiling a 19-month Council of Europe investigation into the complicity of European governments in the practice of whisking terror suspects to secret sites in Europe before deporting them to countries where torture is routine, Dick Marty, a Swiss senator, said senior officials in European governments were fully aware of the policy but continued to cover it up.

"The rendition, abduction and detention of terrorist subjects have always taken place outside the US, where such actions would no doubt have been ruled unlawful and unconstitutional," said Mr Marty's report. "These actions are also unacceptable under the laws of European countries who none the less tolerated them or colluded actively in carrying them out. Some European governments have obstructed the truth and are continuing to do so."

Jerzy Smajdzinski, the Polish defence minister when the renditions occurred, dismissed the Marty findings as "fiction". The Polish government reiterated its denial about hosting secret CIA prisons. But the EU said member states in the firing line were obliged to conduct a full investigation.

Khalid Sheikh Mohammed and another top al-Qaida figure, Abu Zubaydah, were said to have been held at a military intelligence site deep in the forests of northern Poland, flown first to an airfield and then ferried in vans by squads of CIA agents.

Mr Marty said the secret facilities were operated solely by the CIA. "We have sufficient grounds to declare that the highest state authorities [in Europe] were aware of the illegal activities on their territories."

The report from the council, which serves as Europe's principal human rights watchdog, said: "The fight against terrorism must not serve as an excuse for systematic recourse to...massive violation of human rights and contempt for law."

Mr Marty presented his report as the first trial arising from an extraordinary rendition opened in Milan. The former head of Italy's military intelligence service and 25 alleged CIA agents are among those charged with kidnapping a radical Muslim cleric and terrorist suspect, Hassan Mustafa Osama Nasr, known as Abu Omar, who was flown to Egypt where he says he was tortured.

The judge yesterday adjourned the hearings until June 18 when, he said, he would decide whether to suspend the proceedings to wait for a ruling from the constitutional court. Italy's highest tribunal has been asked by the government to block the trial on the grounds that some of the evidence is covered by legislation protecting official secrets.